Kinetics of the Reaction of Ca(OH)2/Fly ash Sorbents with SO2 at High Temperatures
|關鍵字:||飛灰;氫氧化鈣;Ca(OH)2;fly ash||公開日期:||2007||摘要:||本研究以微分反應器，探討飛灰/Ca(OH)2吸收劑在高溫(650~950℃)下之硫酸化反應。吸收劑分為漿化過與未漿化，前者是將飛灰/Ca(OH)2以水固比10/1於溫度65℃下漿化16小時後予以乾燥而得。漿化過飛灰/Ca(OH)2吸收劑含有高比表面積之箔片狀矽酸鈣水合物。在高溫下吸收劑會因燒結而降低反應性，飛灰含量愈多，燒結愈嚴重，且矽酸鈣的反應性因反應平衡限制隨溫度升高而降低。吸收劑的鈣利用率隨SO2濃度增加和反應時間增長而增加，但與溫度的關係則隨吸收劑組成和反應時間而異。吸收劑一小時的鈣利用率在飛灰/Ca(OH)2配比小於約4/6時高於Ca(OH)2本身，但二氧化硫捕捉率則隨飛灰含量增加而下降，而飛灰本身則無反應性。吸收劑在低飛灰含量(配比3/7)時，950℃之鈣利用率高於750℃的鈣利用率，在高飛灰含量(配比7/3)時則相反。配比高於3/7以上時，未漿化之吸收劑之反應性明顯高於經漿化者。本研究使用的兩種飛灰所製備吸收劑以鐵含量較高的飛灰者較佳。含CaSO4和CaSO3吸收劑，鈣利用率及二氧化硫捕捉率較未添加者高。與吸收劑低溫反應之鈣利用率和二氧化硫捕捉率(Ho and Shih,1992,1993; Lin et al.,2003)比較，在低飛灰含量(<40wt%)時，高溫反應的較高，在高飛灰含量時則較低。Ca(OH)2添加適量飛灰(<40wt%)且不必經過漿化處理可以提升其應用於高溫除硫時的利用率。
A differential fixed-bed reactor was employed to study the reaction between fly ash/Ca(OH)2 sorbents and SO2 at high temperatures(650~ 950℃).Slurried and unslurried sorbents were used;the slurried sorbents were prepared by slurrying the solid in water at 65℃ for 16h with a liquid/solid weight ratio of 10/1.Foil-like calcium silicate hydrates were formed in the slurried sorbents. These sorbents have greater surface areas than that of Ca(OH)2or fly ash. But these sorbents tended to sinter at high temperatures, resulting in the decrease of sorbent reactivity,when the fly ash content was high.Also, the reactivity of calcium silicate decreased with increasing temperature due to chemical equilibrium limitation. The utilizations of calcium for sorbents increased with increasing SO2 concentration and reaction time,but the effect of reaction temperature varied with the composition of sorbent and reaction time.The utilizations of calcium at one hour for sorbents were higher than that of Ca(OH)2 alone when the fly ash/Ca(OH)2 weight ratio was less 4/6.However, the SO2 capture per unit weight of sorbent decreased with increasing fly ash content.The fly ash itself was unreactive.The utilizations of calcium for sorbents at 950℃ were higher than those at 750℃ when fly ash content was low(30wt%),but the utilizations of calcium were higher at 750℃ when the fly ash content was high(70wt%).The unslurried sorbents were more reactive than the slurried ones ,especially when the fly ash content was greater than 30wt%.For the two kinds of fly ashes used in this study, the one with a higher Fe content gave sorbents of higher The reactivities. utilization of calciums and SO2 captures were raised when the sorbents were prepared with the addition of CaSO3 and/or CaSO4. Comparing with the low temperature results (Ho and Shih,1992, 1993;Lin et al. ,2003), the utilizations of calcium and SO2 captures were higher for sorbents reacted at high temperatures when the fly ash content was less than 40%.Adding a proper amount of fly ash(<40wt%) to Ca(OH)2 without prior slurrying treatment can raise the utilization of Ca(OH)2 in the high-temperature desulfurization process.
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